The present invention relates to a method of producing a thermistor material or ink which, when screened and fired on ceramic substrates by thick film techniques, exhibits a substantially constant temperature coefficient of electrical resistance. Heretofore in the prior art, most negative temperature coefficient thermistors have exhibited temperature coefficient of resistance (TCR) values which vary depending upon the temperature at which they are measured.
The TCR for any resistor is defined by the following expression: ##EQU1## where .alpha. is the TCR usually expressed in percent per degree centigrade, and R.sub.T is the functional relationship of the sheet resistance upon temperature. If .alpha. is to be constant within the temperature interval T-T.sub.0 and R.sub.T and R.sub.T.sbsb.0 are the resistance values at the respective temperatures, then by integration equation (1) yields: EQU 1n R.sub.T /R.sub.T.sbsb.0 = .alpha.(T-T.sub.0). (2)
Further reduction yields: EQU R.sub.T = R.sub.T.sbsb.0 e.sup..alpha..sup.(T-T .sbsp.0) , (3)
which is the final expression relating resistance to the temperature if the TCR is constant. The resistivities of most thermistor materials vary with temperature according to the following expression: ##EQU2## where both R.sub.T.sbsb.0 and .beta. are constant independent of temperature. Using equation (1) it can be shown that for these materials the TCR depends upon temperature according to the following expression: ##EQU3##